During the evolution of the diurnal lizards, their eyes have lost the typical vertebrate duplex retina with both rods and cones and are instead left only with different types of single and double cones (Röll,
2000; Underwood,
1970; Walls,
1942). However, at some point in evolution a group of lizards, the geckos, turned to a nocturnal lifestyle. In response to the demands of nocturnal vision without rods, the cones of nocturnal geckos have become much larger and more light-sensitive than those of their diurnal relatives (Röll,
2000). Nocturnal geckos have retained three different photopigments sensitive to UV, blue, and green (Loew,
1994) and their eyes are sensitive enough to obtain color information at night (Roth & Kelber,
2004). At intensities corresponding to dim moonlight (0.002 cd m
−2), the nocturnal helmet geckos,
Tarentola chazaliae, could discriminate colors in a behavioral dual choice experiment. At these dim light intensities, their pupils are round and fully opened. In this study, we investigated the pupil, the dimensions of the eye, and the cone dimensions of helmet geckos and used these data to calculate the optical sensitivity with the aim of understanding the adaptations that allow the animals to see colors under dim light conditions.
Eyes adapted for vision at night, such as the eyes of nocturnal geckos, with a large pupil and a short posterior nodal distance (here also called focal length,
f), are especially affected by longitudinal chromatic aberration. As a result, light of short wavelengths is refracted more strongly and thus focused closer to the lens than light of long wavelengths. If this is not corrected for in an eye adapted for nocturnal vision, the retinal image is severely blurred. Multifocal optical systems with distinct concentric zones of different refractive powers have been suggested to correct for some of the defocus on the retina caused by chromatic aberration (Kröger, Campbell, Fernald, & Wagner,
1999). Kröger et al. have shown that the eyes of the nocturnal gecko,
Homopholis wahlbergi, have multifocal optical system. We were interested to know whether the differences between zones of different refractive power match the range of wavelengths the nocturnal geckos are sensitive to.
In addition, the light-adapted pupils in nocturnal geckos are different variations of vertical slit pupils. Apart from the effectiveness in shutting out light during the day, the mode of constriction of slit pupils has been suggested to be of advantage in multifocal eyes, since it allows for all refractive zones of the optical system to be functional at all states of pupil constriction (Kröger et al.,
1999; Malmström & Kröger,
2006). We investigated the pupil dynamics and the multifocal optical system of helmet geckos to see whether the light-adapted pupil allows for all concentric zones of the optical system to refract incoming light.
Some geckos reverted again to a diurnal lifestyle. As a result, the cones of diurnal geckos are small (Röll,
2000) since large photoreceptors are costly and not necessary when light is abundant. In addition, diurnal geckos have small circular pupils and small eyes relatively to body size (Werner,
1969). Since their pupils are small relative to the focal lengths of their eyes, they are less affected by chromatic aberration. Accordingly, previous photorefractometric results show monofocal optical systems in the day gecko,
Phelsuma madagascariensis (Kröger et al.,
1999). We compared the optical system of the day gecko,
Phelsuma madagascariensis grandis, to that of the nocturnal helmet gecko to determine the differences between both species.
One fast method to investigate the optical state of camera-type eyes is photorefractometry. The method makes it possible to study the refractive power in the eyes of living non-cooperative animals from some distance (Schaeffel, Farkas, & Howland,
1987). However, quantitative measurements on the eyes of terrestrial vertebrates are complicated. The refractive power of the cornea is too high to be ignored and lens measurements alone are not sufficient to describe the optical system.
The Hartmann–Shack wavefront sensor has been the most common in-vivo measurement method for ocular wavefront aberrations in human eyes during the last 15 years (Liang, Grimm, Goelz, & Bille,
1994). We have developed a Hartmann–Shack wavefront sensor to obtain quantitative results from gecko optical systems. To allow studies on small eyes, this sensor has a higher resolution than most sensors used on human eyes. However, a common problem occurring in studies of unharmed animals, i.e., limited control of focus and gaze direction (Harmening, Vobig, Walter, & Wagner,
2007; Huxlin, Yoon, Nagy, Porter, & Williams,
2004), might still affect the wavefront analysis.